Forces in Action
Force - interaction between two objects, fields or charges measured in Newtons
Newton - force required to accelerate a mass of 1 kg at a rate of 1 m/s^2
Newton - force required to accelerate a mass of 1 kg at a rate of 1 m/s^2
Resolving Forces
As Galileo proposed, mass and acceleration are two separate entities
Force is the product of these two. As mass is a scalar and acceleration a vector they give a vector.
Giving:
Force (N) F = ma mass (kg) x acceleration (m/s^2)
Force is the product of these two. As mass is a scalar and acceleration a vector they give a vector.
Giving:
Force (N) F = ma mass (kg) x acceleration (m/s^2)
In objects acceleration is generally the changing value as mass remains constant. This however doesn't happen in:
- Rockets - because the fuel is reducing and so the mass reduces
- Approaching the speed of life - The mass of an object increases
Usng the Force equation weight can be calculated by:
Weight (N) W = mg mass (kg) x gravity (m/s^2)
This part explains why a hammer falls faster than a feather
Weight (N) W = mg mass (kg) x gravity (m/s^2)
This part explains why a hammer falls faster than a feather
Non-Constant acceleration
Non-constant acceleration is the reality of the world because objects are subject to drag
Drag depends on:
They will generally ask what happens when someone goes skydiving - the forces in affect are shown below
Drag depends on:
- Velocity
- Cross-sectional area
- Roughness of shape
- Shape
They will generally ask what happens when someone goes skydiving - the forces in affect are shown below
Speed time graph for non-constant acceleration
Drag Force Equation
Fd = Drag Force
p = density of air
Cd = Drag coefficient of object
A = Frontal area
v = velocity
This explains why the acceleration gets harder as terminal velocity is approached.
For when the velocity doubles the drag force quadruples
This also explains why when the parachute on a skydiver is opened the velocity decreases as the drag force increases dramatically
p = density of air
Cd = Drag coefficient of object
A = Frontal area
v = velocity
This explains why the acceleration gets harder as terminal velocity is approached.
For when the velocity doubles the drag force quadruples
This also explains why when the parachute on a skydiver is opened the velocity decreases as the drag force increases dramatically
Equilibrium
Equilibrium is when an object's net force is zero (like at terminal velocity)
Use vector triangles as shown in Motion to work out forces in equilbrium
Centre of Gravity
The point where all weight is said to act (where all the moments are said to act)
To find the centre of gravity of a uniform shape is simple as it's where all the diagonals cross.
To find the centre of gravity of a non-uniform shape is slightly different. You do it by hanging a plumline from different datem points drawing where it falls and finding where they cross.
To find the centre of gravity of a non-uniform shape is slightly different. You do it by hanging a plumline from different datem points drawing where it falls and finding where they cross.
Moments
A moment is the turning effect of a single force acting perpendicular to a beam
The principle of moments states that in a uniform beam in equilibrium balanced on a pivot; the sum of the anti-clockwise forces acting perpendicular to the beam equal the sum of clockwise forces perpendicular to the beam
A moment is measured in Newton metres (Nm) as it is a measure of a force acting at a perpendicular distance from a stated point.
This gives the equation where x is the distance:
sum of clockwise moments Cx = Ax sum of anti-clockwise moments
This gives the equation where x is the distance:
sum of clockwise moments Cx = Ax sum of anti-clockwise moments
In the above diagram (F1 x d1) = (F2 x d2) + (F3 x d3)
Torque
Torque occurs when there is a couple on a beam. This is when two moments work in the same rotational direction at the same perpendicular distance from a point.
Couple = Fs
Density
(density kg/m^3) ρ = m (mass kg)
v (volume m^3)
v (volume m^3)
Pressure
(pressure Pa or N/m^2) p = F (Force N)
A (Area m^2)
Pressure in a liquid
(pressure Pa) p = hρg (height submerged m x density kg/m^3 x gravity m/s^2)
A (Area m^2)
Pressure in a liquid
(pressure Pa) p = hρg (height submerged m x density kg/m^3 x gravity m/s^2)
Car Stopping Distances
Has two parts - thinking distance and braking distance
s = ut + -u^2
2a
s = ut + -u^2
2a
Many different things affect the stopping distance like road condition, concentration of driver etc.
Car Safety
Force felt during a collision is shown in this equation (this isn't given in the exam)
F = m(v-u)
t
F = m(v-u)
t
Crumple Zones
By increasing the time of the impact this decreases the force so crumple zones are designed to increase the time of the impact
Seat Belts
Seat belts prevent you from hitting hard objects in the car by keeping secure in one position
Airbags
Airbags are also designed to increase the time of the impact as they support body parts not protected by the seat belt. Without a seat belt they are dangerous as whilst the body goes forward it hits the airbag traveling towards it, which decreases the time of impact.
The system for an airbag is shown below:
The system for an airbag is shown below:
When the mass experiences 10g (this is only experienced during an accident) it gains enough energy to complete the circuit by touching the connector. This creates the spark needed in the chemical mixture to set off a violent reaction which releases Nitrogen gas which is what expands the airbag.
GPS
Global Positioning Systems work via the principle of Trilateration
The GPS in a car can work out where it is by picking up three satellites signals. Each satellite is in a fixed position above the Earth (in geostationary orbit) and this means that the car can work out how far it from each satellite by picking up the specific satellite signal and working out how far it has travelled. It then plots where it is on a map.
The GPS in a car can work out where it is by picking up three satellites signals. Each satellite is in a fixed position above the Earth (in geostationary orbit) and this means that the car can work out how far it from each satellite by picking up the specific satellite signal and working out how far it has travelled. It then plots where it is on a map.